1. Field of the Invention
The present invention relates to a method for producing a Ga-containing group III nitride semiconductor having reduced threading dislocation.
2. Description of the Related Art
A method of forming a low temperature buffer layer on a sapphire substrate by a metal organic chemical vapor deposition method (hereinafter referred to as an “MOCVD method”) and growing GaN on the buffer layer has conventionally been known.
For example, in JP-A-2005-19872, a sapphire substrate is heat-treated at 1,135° C. to clean the surface thereof, a substrate temperature is decreased to 515° C., a buffer layer comprising GaN having a thickness of 20 nm is formed, the substrate temperature is increased to 1,075° C., and fine crystals of GaN are formed on the sapphire substrate. Thereafter, the temperature is maintained, hydrogen gas ratio is made larger than nitrogen gas ratio, and GaN is facet-grown using fine crystals of GaN as nuclei. Thereafter, the substrate temperature is decreased to 1,005° C., nitrogen gas ratio is made larger than hydrogen gas ratio, making it easy to grow in a horizontal direction, and GaN is grown so as to fill spaces among facets. Thus, GaN having reduced threading dislocation density is obtained.
In Example 3 of JP-A-2005-183524, a sapphire substrate is thermally cleaned at 1,200° C., a substrate temperature is set to 1,200° C., AlN is epitaxially grown, and a monocrystal underlying layer 102 having a thickness of 0.7 μm is formed. The substrate temperature is decreased to 1,150° C., an AlGaN layer 103 is epitaxially grown in a thickness of 100 nm or less, and an annealing treatment is conducted by maintaining the substrate temperature at 1,350° C. for 10 minutes. The substrate temperature is then decreased to 1,150° C., and an AlGaN layer 104 is grown. Thus, threading dislocation density of the AlGaN layer is reduced.
However, the method of JP-A-2005-19872 is a method that after forming a low temperature buffer layer comprising ultra thin GaN having a thickness of 20 nm at low temperature, the temperature is increased to a temperature capable of growing GaN to form fine crystals of GaN, and subsequently, GaN is facet-grown. Therefore, after formation of the low temperature buffer layer, the temperature of the substrate is merely increased up to a temperature capable of growing GaN, and therefore, crystal nuclei of fine crystals are small. As a result, density of occurrence origin of threading dislocation is still large.
The method of JP-A-2005-183524 is that an underlying layer 12 on a substrate 11 is epitaxially grown in a thickness of 0.7 μm, and is therefore fine crystal. Furthermore, the AlGaN layer 103 is epitaxially grown on the underlying layer 102 of fine crystal, and is therefore fine crystal. Annealing at a stage that the AlGaN layer 103 has been formed facilitates movement of dislocation in the AlGaN layer 103, thereby reducing threading dislocation (paragraph 0032).
Therefore, P-A-2005-183524 does not increase crystal nuclei in a buffer layer in a polycrystal, amorphous or polycrystal/amorphous mixed state, and does not inhibit formation of threading dislocation density in an objective semiconductor layer to be grown.
JP-A-2005-19872 relates to a heat treatment for obtaining crystal nuclei for facet growth, and does not decrease density of occurrence origin of threading dislocation.